We can only account for 4 per cent of the cosmos

IN 1997, astronomers discovered that the universe is expanding at ever faster speeds. No one knows why this should be: until that point, everyone assumed the universe's expansion would be slowing down after the big bang. The best explanation we have – that some mysterious thing called “dark energy” is causing the accelerated expansion – is no explanation at all because we have no idea what this dark energy actually is.

We can say the same about “dark matter”. If you take our best understanding of gravity, apply it to the way galaxies spin, and you'll quickly see the problem: the galaxies should be falling apart. Galactic matter orbits around a central point because its mutual gravitational attraction creates centripetal forces. But there is not enough mass in the galaxies to produce the observed spin. The best response from physicists is to suggest there is more stuff out there than we can see, and that the gravity of this stuff is holding everything together. Just as they called the mysterious accelerating stuff dark energy, they call this mysterious gravitating stuff dark matter. We’ve been searching for dark matter for decades now, but we still have no idea what it might be.

That may be because dark energy and dark matter, which together seem to make up 96 per cent of the universe’s contents, might be a cosmic mirage. We may find out that the dark energy vanishes in a puff of logic when we rid our mathematics of certain assumptions – such as the assumption that the universe is the same in every direction. Dark matter might be a result of us not quite understanding how gravity works. The issue is still wide open.